This invention relates to vehicle drive systems and more particularly to systems in a vehicle having more than one engine and requiring a coordination of the separate transmissions of the vehicle.
To provide increased traction and load-carrying capacity, certain vehicles are provided with more than one engine in an arrangement wherein each engine drives a separate set of wheels, tracks or other ground-engaging members through separate transmissions. An example of such a vehicle is a tractor-drawn scraper wherein one engine and its transmission is mounted on the tractor unit for delivering power to the tractor wheels and wherein a second engine and its transmission are mounted on the scraper unit to deliver power to the scraper wheels.
During the loading phase, which occurs at vehicle speed between two and four miles per hour, it is essential to provide sufficient torque to the tractor and scraper wheels over this range of speeds which does not require shifting of the transmission in the event the scraper encounters varying resistances. Typically this is accomplished by coupling the two engines to their respective wheels by means of torque converters when the transmissions are in their lowest range.
At the higher vehicle speeds at which the loaded scraper is run when transporting earth, the transmissions are conditioned for straight mechanical drive, and the speed ranges available in the various gears are so selected that a minimum horsepower gap is produced between steps. Accordingly, the output horsepower of the system is more uniform throughout the various high speed ranges.
A typical tractor engine will utilize a transmission having seven or so forward gear ranges, as well as a reverse gear range.
It is important that the tractor wheels and scraper wheels be driven at as nearly the same speed as possible throughout the range of operation. If this is not done, then the difference in speed of the two sets of wheels will cause one set to slip relative to the other and thereby cause undue tire wear.
Typically, the problem of wheel slippage is met by providing a transmission for the scraper unit which is identical to that for the tractor unit, the two transmissions having the same number of gear ranges. The two transmissions are then simultaneously shifted through ranges so that the speeds of the two sets of wheels are varied in unison. Although such use of identical transmissions does provide for speed matching, this increases the cost of the vehicle since such transmissions are quite complex and corresponding expensive to build and install. Examples of planetary gear transmissions used in earth moving machines are shown in U.S. Pat. Nos. 33,381,546 and 3,347,113.
It is a main object of the present invention to provide a transmission system for a dual engine-powered vehicle wherein one of the transmissions has a considerably lesser number of gear ranges than the other but wherein the two transmissions provide proper speed matching throughout the range of operation of the vehicle.
In order to further reduce costs, it is another main object of the invention to provide such a transmission system wherein a substantial portion of the transmission with the smaller number of gear ranges is identical to a portion of the other transmission.